Abstract
A given dose of ultraviolet irradiation produces as many pyrimidine-dimer-containing photoproducts in the DNA of the Escherichia coli Rec − strain JC1569 as it does in the related Rec + strain JC1557. Both strains excise dimer-containing photoproducts from their DNA to the same extent. In comparison with JC1557, approximately 30 times as much DNA of the Rec − strain is degraded per unit dose of ultraviolet light. Perhaps as a result of this excess ultraviolet-induced degradation, irradiated Rec − cells do not incorporate appreciable amounts of exogenous thymidine or thymine into DNA and have few survivors following exposure to an ultraviolet dose which almost all Rec + cells survive. A numerical argument is offered to establish the ability of Rec − cells successfully to replace excised pyrimidine-dimer-containing regions of the DNA with normal nucleotide residues. No effect of the rec − mutation in JC1569 was found on the level of activity of the following enzymes: endonuclease I, exonuclease I, exonuclease III and DNA phosphatase, and DNA polymerase and exonuclease II.
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